FORMULA: n(n+1)/2 =>asta doar atunci cand ai 1+2+3+...+n
a)5+10+15+...+45=5(1+2+3+...+9)=5*9(9+1)/2=45*10/2=450/2=225
b)7+14+21+...+63=7(1+2+3+...+9)=7*9(9+1)/2=63*10/2=630/2=315
c)8+16+24+...+72=8(1+2+3+...+9)=8*9(9+1)/2=72*10/2=720/2=360
d)11+22+33+...+99=11(1+2+3+...+9)=11*9(9+1)/2=99*10/2=990/2=495
